Re: [Caml-list] Only bytecode version of executable allocating huge amounts

[Xavier Leroy <xavier.leroy@college-de-france.fr>]

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On Tue, May 28, 2019 at 2:27 PM Jon French <jf451@cam.ac.uk> wrote:

> Hi all,
>
> I'm wondering if anyone on this list might have encountered this issue
> before, since it's got me pretty stumped.
>
> We have an OCaml project ( https://github.com/rems-project/sail ) which
> tries to allocate huge amounts of memory, but only when compiled to
> bytecode. The native version works perfectly fine.
>

This is unusual indeed.

Do you have a repro case that you could share publicly?  If so, please
submit an issue at https://github.com/ocaml/ocaml/issues and attach the
repro.

The first thing I would do is to increase the verbosity of the GC: set the
OCAMLRUNPARAM environment variable to the value "v=255".  The resulting GC
traces can give an idea of what's going on.

Kind regards,

- Xavier Leroy




>
> strace for an example run ends in:
>
> openat(AT_FDCWD, "/home/ojno/work/sail2/lib/vector_dec.sail",
> O_RDONLY|O_CLOEXEC) = 3
> lseek(3, 0, SEEK_CUR)                   = 0
> read(3, "$ifndef _VECTOR_DEC\n$define _VEC"..., 65536) = 7031
> mmap(NULL, 1965819695104, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
> -1, 0) = -1 ENOMEM (Cannot allocate memory)
> brk(0x5729f32c4000)                     = 0x55603f2f4000
> mmap(NULL, 1965819826176, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
> -1, 0) = -1 ENOMEM (Cannot allocate memory)
> write(2, "Fatal error: out of memory.\n", 28Fatal error: out of memory.
> ) = 28
> exit_group(2)                           = ?
> +++ exited with 2 +++
>
>
> which naturally fails since I don't have 2 TB of memory available.
>
> The exact amount of the allocation varies from input to input and even run
> to run, but is always that approximate size. There's no characteristic
> pattern of heap or stack running out of control if I turn on memory
> debugging info in $OCAMLRUNPARAM, just that one huge allocation. When run
> in ocamldebug, it also doesn't appear to be failing at a location which is
> sensible to be allocating such amounts of memory -- and the location also
> varies with the input. And on tiny inputs it doesn't seem to try and make
> the allocation at all.
>
> Are we actually seeing a compiler/runtime bug here? Is there something I'm
> missing?
>
> Thanks,
>
> Jon French
> Computer Laboratory
> University of Cambridge
>

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